Display screen or portion thereof with a graphical user interface



FIG. 1 is a front view of a first embodiment of a display screen or portion thereof with a graphical user interface showing the new design; and,

FIG. 2 is a front view of a second embodiment thereof.

The outermost broken line rectangle illustrates a display screen or portion thereof and forms no part of the claimed design. The remaining broken lines illustrate portions of the graphical user interface that form no part of the claimed design. 

CLAIM I claim the ornamental design for a display screen or portion thereof with a graphical user interface, as shown and described. 